1. Field of the Invention
The present invention relates to a polarizing screen for use with a video display system equipped with a liquid crystal display device and, more particularly, to the polarizing screen of a type having a polarizing capability of selectively transmitting therethrough or reflecting therefrom the incoming light depending on the direction of polarization.
2. Description of the Prior Art
In general, the liquid crystal video display system, that is, the video display system utilizing a liquid crystal display device, has the following advantages as compared with the CRT video display system utilizing a cathode ray tube.
(1) Due to the compact feature, the liquid crystal video display system can be installed at a relatively small space.
(2) Due to the light-weight feature, the liquid crystal video display system can be transported from place to place easily.
(3) Due to the digital images, the image processing is carried out favorably.
However, the conventional liquid crystal video display system involves an increased amount of loss of light and a low efficiency of utilization of light. This is because rays of light emitted from a source of light is complicatedly processed in the following manner. The rays of white light emitted from the light source made of metal halide is first separated into three light beams of three primary colors by means of a dichroic mirror. The light beams are projected onto a liquid crystal light valve after having been converted into linearly polarized light by means of a linear polarizing element of a light absorbing type. On the other hand, a voltage modulated with a video signal is applied to the liquid crystal light valve to rotate the polarizing plane (the plane of electromagnetic vibration) of the linearly polarized light so that, as the light projected onto the liquid crystal light valve passes therethrough and subsequently pass through an analyzer element (another linear polarizing element), components of rays of light corresponding to the angle of rotation of the polarizing plane are projected onto a screen to thereby form a visible image.
Accordingly, with the prior art screen used in connection with the conventional liquid crystal video display system, the contrast of the projected image is considerably low where the screen is used under the environment with ambient light such as illuminating light considerably reflecting from the screen and, therefore, particularly where the screen of a relatively large size is utilized to present images projected through the liquid crystal video display system, viewers are required to view them in a dark room.
In view of the foregoing, the following techniques have been suggested as a means for increasing the contrast of the projected picture on the screen by decreasing the amount of light originating from ambient light such as, for example, an illuminator lamp and, yet, without accompanying a loss of the imagewise rays of light. Specifically, U.S. Pat. No. 2,362,573 discloses the minimization of emission of external rays of light incident upon the screen by the utilization of a laminated structure of a polarizing element and a quarter-wavelength plate; U.S. Pat. No. 2,793,361 discloses a technique in which a source of illuminating light is covered by a polarizing element while the screen is covered by another polarizing element of a type having a plane of electromagnetic vibration of transmissive polarized light lying perpendicular to that of the polarizing element covering the illuminating light source; U.S. Pat. No. 3,319,519 discloses a technique in which a source of imagewise rays of light is covered by a polarizing element while the screen is covered by another polarizing element of a type having a plane of electromagnetic vibration of transmissive polarized light matching with that of the polarizing element covering the source of imagewise rays of light; and the Japanese Laid-open Patent Publication No. 50-98831 discloses a technique in which, not only are a source of imagewise rays of light and a screen covered, respectively, by a first polarizing element and a second polarizing element of a type having a plane of electromagnetic vibration of transmissive polarized light matching with each other, but also a source of illuminating light is covered by a third polarizing element having a plane of electromagnetic vibration of transmissive polarized light matching with that of any one of the first and second polarizing element.
Other references of interest include the Japanese Laid-open Patent Publication No. 51-71697 which discloses the utilization of a liquid crystal video image analyzing element on a surface of the screen; the Japanese Laid-open Patent Publication No. 3-287244 which discloses a technique in which an array of polarizing beam splitters having a plane of electromagnetic vibration of transmissive polarized light matching with linearly polarized light carrying an image as a result of passage through a liquid crystal display device is disposed on a surface of the screen; the Japanese Laid-open Patent Publication No. 4-63333 which discloses a technique in which a polarizing element having a light absorption characteristic adapted to a dispersion of wavelengths of three primary colors formed by respective liquid crystal panels is disposed on a surface of the screen; and the Japanese Laid-open Patent Publication No. 4-29127 which discloses a technique in which an array of minute polarizing elements and minute light absorbent elements alternating with each other is disposed on a surface of the screen.
On the other hand, in order to realize a screen for the presentation of the image, formed by a liquid crystal video projector, in a manner required for one or more viewers to clearly view the projected image, not only is it required to increase the contrast of the projected image, but also the screen should be of a large format size sufficient to provide a large format picture and, at the same time, the screen must be so flexible as to be rolled up for storage or transportation. In view of this, as means for improving overall aspects of the screen to satisfy the various requirements, formation of a screen with the use of a woven polarizing fabric having a function of an array of polarizing lenslets has been developed such as disclosed in, for example, the Japanese Laid-open Patent Publication No. 5-197026 corresponding to U.S. Pat. No. 5,251,065.
It has, however, been found that the polarizing screen utilizing the woven fabric requires some improvements to be done in such a way as to increase the efficiency of utilization of the imagewise rays of light by increasing the efficiency of removal of the illuminating light and, also, by increasing the luminance of the projected image within a predetermined viewing angle.